Dienes, especially isoprene, are useful as monomers for the manufacture of synthetic rubbers. Several fundamental processes have been used to construct the isoprene C.sub.5 skeleton from smaller carbon units. These processes are not commercially accepted in that there are numerous problems associated with each particular synthesis route.
European Patent Application No. 80,449 discloses the synthesis of isoprene from linear butenes wherein mixed linear butenes are catalytically isomerized to a mixture of cis and trans-butene-2 and then hydroformylating the butene-2 mixture to 2-methylbutanal (2MBA) in the presence of a homogeneous rhodium catalyst and an organic ligand. The 2MBA is then dehydrated to isoprene in the presence of an acidic heterogeneous catalyst at elevated temperatures. The preferred catalyst for the dehydration step is a boron phosphate catalyst which is described in British Patent No. 1,385,348. The dehydration reaction is endothermic, and under preferred conditions, the reaction is performed in the vapor phase over a fixed bed of catalyst at elevated temperatures from about 200 to about 400.degree. C. This reference does not disclose the length of time the catalyst performs at such selectivities and conversions (lifetime). Commercial production of isoprene via the aldehyde dehydration route has not been established since the dehydration catalyst is known to have short lifetimes which limit its utility in commercial applications.
U.K. Patent No. 1,385,348 relates to the conversion of aldehydes to dienes with conjugated double bonds. This British patent discloses that the preferred acid dehydration catalysts are mixed acid anhydrides, for example, boron phosphate, silica borate or silica titanate. There is no disclosure in the U.K. patent regarding the duration of conversions and selectivities and/or the number of regenerations required during any particular time period.
U.K. Patent No. 2,093,060 relates to the preparation of dienes, especially isoprene, from a corresponding carbonyl compound in which magnesium ammonium phosphate or its decomposition products are used as the dehydration catalysts.
U.S. Pat. No. 4,524,233 discloses an improved boron phosphate catalyst wherein the catalyst is prepared by combining phosphoric acid and boric acid at such molar ratios that the ratio of P/B is less than 1 but more than 0.6. The boron phosphate is then contacted with from 0 to 10 mole percent based on moles of boron with ammonia or an amine. The thus treated boron phosphate is then calcined in air and steamed prior to its use. U.S. Pat. No. 4,524,233 also suggests that the 2MBA feed to the dehydration reactor may be diluted with a solvent such as pentane, hexane, heptane, octane and nonane to lessen the deactivation of the catalyst in the dehydration reaction. U.S. Pat. No. 4,524,233 also suggests that the boron phosphate catalysts have incorporated therein from 0.1 to 10 weight percent graphite to further lessen the deactivation of the catalyst.
A disadvantage associated with known catalysts to dehydrate aldehydes is that catalyst life depends on many factors which include catalyst composition, structure, activity, operating temperatures, and coke deposition. Coke deposition is understood to denote coke (carbonaceous) deposits formed on the catalyst during the dehydration reaction.
The use of boron phosphate as a catalyst for the dehydration of alcohols such as 2-butanol and 2-methyl-2-butanol is known. See Jewur and Moffat, Journal of Catalysis, 57, 167-176 (1979). The problems associated with an aldehyde dehydration are different and more difficult to overcome than those found in alcohol dehydrations. For example, the boron phosphate dehydration of 2-methylbutanol yields only 2-methyl-2-butene and 2-methyl-1-butene, while dehydration of 2MBA yields primarily methylisopropyl ketone, 2-methyl-2-butene, 2-methyl-1-butene and isoprene. It is the production of the conjugated diolefin, isoprene, that makes the aldehyde dehydration so difficult, since this highly reactive monomer is known to form dimers and/or polymerize in the presence of acid catalysts.
In addition, aldehydes such as 2MBA are known to undergo aldol condensation. This is a reaction between two molecules of an aliphatic aldehyde whereby a 3-hydroxyaldehyde is formed. Dehydration of a 3-hydroxyaldehyde results in the formation of terpenes, a highly undesirable by product that can coke and deactivate the catalyst. Due to these and other differences, catalysts and processes suitable for long term dehydration of alcohols have not been found acceptable for aldehyde dehydrations.
One aspect of this invention is directed to the use of an additive to the aldehyde feed. The instant invention has utility with any catalyst known to promote the dehydration of an aldehyde to a diolefin. The prior art does not suggest, disclose or appreciate that the presence of a specific aromatic compound in the aldehyde feed will unexpectedly lengthen the viable lifetime of the catalyst. In addition, it has been found that the benefits of the instant invention will continue even after the addition of the aromatic compound to the aldehyde feed has ceased, thus allowing for intermittent addition of the aromatic compound to the aldehyde feed.
A portion of the instant invention is thus directed to a process for the catalytic dehydration of an aldehyde to a diene which will allow the catalysts an extended lifetime of high selectivity and low coke deposition. The prior art does not suggest or disclose an additive to the aldehyde feed which would make the dehydration of aldehydes to diene suitable for commercial application.